1. Field of the Invention
The present invention relates to a method of transmitting, and more particularly, to a method of transmitting using phase shift-based precoding and an apparatus for implementing the same in a wireless communication system.
2. Discussion of the Related Art
With respect to wideband code division multiple access (W-CDMA) systems, researches are being conducted using a multiple antennas to increase system capacity, transmit speed of data, and link reliability by way implementing various schemes such as beamforming, multi-input, multi-output (MIMO), and transmit diversity. In particular, the MIMO scheme promotes high speed transmission via spatial diversity, similar to V-BLAST, is adopted in a 3rd Generation Partnership Project (3GPP).
Furthermore, the two (2) antenna system adopted in Release 99 and Release 4, based on transmit diversity, has been improved to a new type of diversity scheme, such as a per antenna rate control (PARC) or a per user unitary rate control (PU2RC), which considers operation using more than three (3) antennas.
FIG. 1A is an exemplary diagram illustrating a structure of a PARC for a single user. FIG. 1B is an exemplary diagram illustrating a structure of a PARC for multiple users.
With respect to conventional V-BLAST, each transmit antenna can be configured using the same modulation and encoding without feedback information from channel quality information (CQI). However, as shown in FIGS. 1A and 1B, the PARC uses the feedback information regarding the channel conditions, such as a modulation coding set (MCS) and/or a transmit antenna subset (TAS), and selects the user data stream to be transmitted by each antenna.
Referring to FIG. 1A, any one of the three (3) user data streams is selected since this is an exemplary illustration the PARC for single user. Referring FIG. 1B, at least two (2) of the three (3) user data streams are selected since this is an exemplary illustration the PARC for multiple users.
Thereafter, the modulation and encoding using the feedback information regarding the channel conditions is applied to the user data streams stored in the buffer after being demultiplexed. The user data streams are then multiplexed using a scheme (e.g., orthogonal frequency division multiple access (OFDMA)) and transmitted via each antenna.
In other words, a base station (BS) applying the PARC scheme uses the feedback information transmitted from a mobile station (MS) to perform scheduling for optimizing transmission rate. Through this, one MS or two or more MSs can simultaneously share frequency and time resources in the space domain. Moreover, the PARC scheme allows for increase in diversity gain as a number of MSs scheduled by the BS increases.
By using the PARC scheme, the feedback overhead is reduced since only the CQI is used as the feedback information. With smaller or reduced overhead, there is relatively less possibility of error during the feedback process, and switching can take place between the PARC for the single user and the PARC for the multiple users. However, in case of the PARC for multiple users, interference between users can occur thus affecting transmission efficiency.
FIG. 2 is an exemplary diagram illustrating a structure of a PU2RC. The PU2RC uses spatial multiplexing for transmitting data streams of multiple users. As such, multiple data streams are selected for transmission to multiple users. In the PU2RC, a unitary matrix based on a singular value decomposition of the MIMO channel is used to perform precoding.
More specifically, the unitary matrix in a transmitter is a set of unitary basic vectors selected by all users (or MSs). If the set of vectors is fixed, represented by M, the unitary basic vectors are selected by one or multiple users.
Furthermore, the PU2RC can be used to reduce inter-user interference and achieve high efficiency gain. However, the feedback information size can be large since information can include preferred matrix index in addition preferred vector in the matrix, thus increasing the possibility of transmission error due to the large size of the feedback information